The dir-as-disk feature seems to corrupt the data sometimes, when the directory contains more than 720KB of data.
Steps to reproduce the problem:
1) Put both the MSX-DOS1 and MSX-DOS2 files (MSX-DOS?.SYS and COMMAND?.COM)
2) Add some files to the same dir, making it pass the 720KB limit
3) Boot a MSX Turbo-R. It will boot the MSX-DOS2
4) Modify some file
5) Shut down the MSX Turbo-R
6) Boot a Panasonic FS-A1WSX. It will boot the MSX-DOS1
7) The MSX-DOS1 is probably corrupted. It freezes after the "MSX-DOS version 1.0" message.